One of the greatest environmental threats today is the continuous increase of carbon dioxide in the atmosphere. Some of the greatest man-made carbon dioxide emissions are caused by flue gases of power plants using fossil fuels (coal, natural gas, oil, peat). The rationale for the use of current recovery methods is not so much the climate change, but carbon dioxide is separated and purified from flue gases for industrial use and raw materials of industrial downstream processes.
According to a study by Li et al. 2011 (Li Yongling, Liu Yingshu, Zhang Hui & Liu Wenhai 2011. Carbon dioxide capture technology. Energy Procedia 11 (2011), p. 2508-2515.), the most important methods, currently at least in the pilot stage, for separating carbon dioxide from flue and process gases, are:
1) Chemical absorption in solutions, such as amine or hydroxide solutions [(Ma'mun Sholeh 2005. Selection and characterization of new absorbents for carbon dioxide capture. Faculty of Natural Science and Technology Department of Chemical Engineering, Norwegian University of Science and Technology (NTNU), Doctoral thesis. 132 p) and (Chakravarti Shrikar et al. 2001. Advanced technology for the capture of carbon dioxide from flue gases. First National Conference on Carbon Sequestration, 15-17 May 2001, Washington, D.C. The National Energy Technology Laboratory (NETL). 10 p.)], in liquids, such as ionic liquids, or in solid materials, such as carbonation of calcium or lithium,
2) Physical absorption in solutions or liquids, such as modified ionic liquids,
3) Adsorption on solid surfaces [(Munoz Emilio, Diaz Eva Ordonez Salvador & Vega Aurelio 2006. Adsorption of carbon dioxide on alkali metal exchanged zeolites. 2006 Aiche Annual Meeting, 12-17 Nov. 2006, San Francisco, Calif., paper 71.) and (Bonenfant Danielle et al. 2008. Advances in principal factors influencing carbon dioxide adsorption on zeolites. Science and Technology of Advanced Materials 9 (2008) 013007, 7 p.)],
4) Membrane separation processes [(Feron & Jansen A. E. 1997. The production of carbon dioxide from flue gas by membrane gas absorption. Energy Convres. Mgmt 38 (1997), p. 93-98.) and (Zhikang Xu et al. 2001. Separation and fixation of carbon dioxide using polymer membrane contactor. First National Conference on Carbon Sequestration. 15-17 May, 2001, Washington, D.C. The National Energy Technology Laboratory (NETL). 8 p.) and (Ho Minh T., Allinson Guy W. A. & Wiley Dianne E. 2008. Reducing the cost of CO2 capture from flue gases using membrane technology. Ind. Eng. Chem. Res. 47 (2008), p. 1562-1568.)] and
5) Cryogenic distillation.
The greatest challenge of separation methods is the high flow of flue or process gas, which increases the circulation rate of sorbents, equipment dimensions and energy consumption, for example. Problems are also created by the need and consumption of chemicals, their processing costs and, on the other hand, difficulties in scaling up. For example, the membrane method has so far been applied only on a small scale. The use of certain chemicals, such as amines, can cause adverse health and environmental effects.
The amine technology, based on chemical absorption, is the most commonly used method for producing carbon dioxide from flue gases for various industrial applications. In the amine-based absorption process, amine reacts with carbon dioxide to produce carbamates. In a typical amine-based absorption process, flue gas must first be cooled and impurities (particles, SOx, NOx) must be removed from it to obtain a tolerable level [Cousins A., Wardhaugh L. T. & Feron P. H. M. 2011. A survey of process flow sheet modifications for energy efficient CO2 capture from flue gases using chemical absorption. International Journal of Greenhouse Gas Control (2011), 15 p. (in press)]. Purified and cooled flue gas is blown through an absorption column in a countercurrent direction towards a lean amine solution. The columns are typically randomly packed columns, and their dimensions may be very large. In packed columns developed during recent years, it has been possible to minimize pressure drops during gas blowdown [Menon Abhilash & Duss Markus 2011. Sulzer-reducing the energy penalty for post-combustion CO2 capture. Carbon Capture Journal (2011) 23, p. 2-5.].
The use of the amine-based process is most of all limited by the heating costs of amine regeneration. Heating is carried out with water vapor at 3 bar minimum for which the specific energy consumption ranges from 3.8 to 5.3 MJ/kg CO2, or from 1.1 to 1.5 MWh/t CO2, and the consumption of electricity of pumps and fans ranges from 0.1 to 0.3 MWh/t CO2, depending on the process [Lee Young, Kwak No Sang & Lee Ji Hyun 2012. Degradation and corrosivity of MEA with oxidation inhibitors in a carbon dioxide capture process. Journal of Chemical Engineering of Japan, Advance Publication. 20 Jan. 2012. 5 p.]. The steam consumption ranges between ¼ and ⅓ of the steam production of a coal-fired power plant [(Yang Hongqun, Xu Zhenghe, Fan Maohong, Gupta Rajender, Slimane Rachid B, Bland Alan E & Wright Ian 2008. Progress in carbon dioxide separation and capture: A review. Journal of Environmental Sciences 20(2008), p. 14-27.) and (Dave N., Do T., Palfreyman D., Feron P. H. M., Xu S. & Gao S. & Liu L. 2011. Post-combustion capture of CO2 from coal-fired power plants in China and Australia, an experience based cost comparison. Energy Procedia 4 (2011), p. 1869-1877.) and (Karmakar Sujit & Kolar Ajit Kumar 2011. Thermodynamic analysis of high-ash coal-fired power plant with carbon dioxide capture. International Journal of Energy Research (2011) (abstract))]. A major part of this steam is not available for power production. Impurities in flue gas and particularly oxygen contained in flue gas can cause corrosion and decomposition of chemicals. Impurities originating from flue gas and chemicals must be continuously removed from the amine solution.
A problem with the amine-based process is related to the environmental and health risks associated with it. Amines and their decomposition products may be toxic to man, animals and aquatic organisms. For example, decomposition products of amines, nitroamines [Fostås Berit, Gangstad Audun, Nenseter Bjarne, Pedersen Steinar, Sjøvoll Merethe & Sørensen Anne Lise 2011. Effects of NOx in the flue gas degradation of MEA. Energy Procedia 4 (2011), p. 1566-1573.] may cause cancer or pollute drinking water. All of the decomposition products of amines and their effects are not yet known. The effects depend on the amine type used [Botheju Deshai, Li Yuan, Hovland Jon, Haugen Hans Aksel & Bakke Rune 2011. Biological treatment of amine wastes generated in post combustion CO2 capture. Energy Procedia 4 (2011), p. 496-503.]. Amine emissions can be carried to the atmosphere or waterways in liquid or gaseous form. The estimated quantity of amine emissions ranges from 300 to 3000 t/a, whereas the amount of generating carbon dioxide is 1 Mt/a. [Shao Renjie & Stangeland Aage (Bellona Foundation) 2009. Amines used in CO2 capture—health and environmental impacts. Bellona Report, September 2009. 49 p.]
In physical absorption, carbon dioxide is dissolved in a solution or liquid at a high pressure and released by decreasing the pressure and/or increasing the temperature. Physical absorption is the most efficient when the absorption pressure is high and the temperature is low. In flue gases, the partial pressure of carbon dioxide is relatively low, generally between 5% and 15% in a flue gas at normal pressure. An absorption process based merely on a physical mechanism requires pressurization of gas to achieve good absorptive capacity. The lower the pressure at which absorption can be carried out, the smaller the requirement of electricity-consuming pressurization energy. In flue gas pressurization, more than 80% of energy is consumed for nitrogen pressurization. Solutions used in physical absorption include (poly)propylene carbonate (PC), methanol, N-formylmorpholine and N-acetylmorpholine (NAM) (product name Morphysorb®), N-alkylpyrrolidone, such as N-methylpyrrolidone (NMP), sulfolane, polyethylene glycol ethers (e.g. Selexol®), glycerols, e.g. glycerol carbonate and liquid carbon dioxide [Aho Yrjö 2009. Menetelmä hiilidioksidin talteenottamiseksi savukaasuista. Patent application FI20085233, 19 Sep. 2009. 11 p.].
The physical absorption process is today used in applications in which the partial pressure of carbon dioxide is high, for example, in natural gas purification. In physical absorption, carbon dioxide does not react with the solvent; therefore, solvent regeneration is easier than in chemical absorption.
Equipment used in the thesis (2007) of Jussi Lähtelä “Kaatopaikkakaasun puhdistaminen liikennepolttoaineeksi vastavirtavesiabsorptiolla” is also known as prior art, wherein an absorption column based on the countercurrent principle and desorption are used for separating carbon dioxide from biogas. In physical absorption, water is used as the solvent in the absorption column. Due to the poor absorptive capacity of water, the supply pressure of biogas fed to the absorption column must be increased to a remarkably high level. With increasing pressure, the solubility of carbon dioxide in water increases and thus the efficiency of absorption increases. However, the operating costs of the process increase at the same time. In the desorption column, carbon dioxide and other gases dissolved in water are separated from water and released into the atmosphere. The equipment used in Lähtelä's thesis is not designed for the recovery of carbon dioxide, and neither can it be economically used for this purpose.